Solid state photoresistors or photodiodes are frequently applied in photometers and spectrophotometers as sensing elements to detect radiation. These sensors are connected to a suitable measuring circuit so that the output voltage of the resulting intensity measuring unit should be proportional to the radiation to be measured. However, the dark resistance of the photoresistors, i.e. the measurable resistance of unradiated photresistors is not of infinite but of finite value depending also on temperature. Likewise the dark current of photodiodes is not zero either, but of finite value depending also on temperature. Dark resistance and dark current, respectively, is effected also by nuclear radiation (.alpha., .beta., .gamma.) acting on the sensor; furthermore the value of these parameters is subjected to permanent changes during the useful lifetime of these devices. Consequently the output voltage of the aforementioned intensity measuring unit is not zero even if the sensor capable of detecting radiation is not subjected to radiation at all, but it has a small finite value slightly changing in time. In case of measuring a spectrum the useful signal is superimposed onto this slowly changing zero level signal. The voltage detected at the output of the intensity measuring unit therefore includes the momentary value of the zero level voltage added to the voltage characterizing the intensity of the radiation to be measured.
Since the intensity of the measured radiation is characterized only by the latter, an effort is made to establish the value of the zero level voltage and to subtract it--automatically, if possible--from the output voltage; thereby the resulting differential voltage will only be characteristic to the intensity of the radiation to be measured. A wide-spread solution to this problem is to intermittently cut the beam path of the spectrometer and then to measure the value of the zero level voltage on the output of the intensity measuring unit; this value is stored by analog means and is subtracted from the output voltage of the amplifier during the measuring period following the opening of the beam path. Another solution is also known whereby a suitable voltage or current is applied to the input of the amplifier connected to the sensor while the diaphragm cuts off the beam path so that the output voltage should be set to zero and then, during the subsequent measuring period this voltage or current stored by analogue means remains applied to the input; thereby the output voltage will characterize only the intensity of the measured radiation acting upon the sensor.
These methods apparently eliminate the error resulting from the variations of zero level signal. Acutally the error partly remains, and if the conditions are unfavourable an undesirable additional error can even occur. The former error can be attributed to theoretical, the latter to practical reasons. The residual error attributed to theoretical reasons is the result of the fact that the output voltage compensated at the beginning of the measuring period would only remain zero during the measuring period if the dark signal of the sensing element did not change meanwhile. Practically, however, the dark signal and thereby the zero level signal is even subjected to variations in the interval of the measuring period and this variation results in a measuring error. The undesirable additional error is the result of the analog method of storing the voltage necessary to compensate the output, namely a capacitor charged to the required voltage to store it during the next measuring period. This storage can however be of limited accuracy, since some of the charge will be lost partly as a result of the conductivity of the capacitor and partly as a result of the finite resistance of the circuits connected. Consequently, the output voltage accurately zeroed at the beginning of the measuring period would differ from zero at the end of the measuring period even if the dark signal of the sensing element did not change meanwhile.
There is only one condition for the zero output voltage not to change during the measuring period: when the errors resulting from the loss of charge in the capacitor and the variations of the zero level are equal and of opposite polarity. This can only occur occasionally and within a relatively short interval; it can not be relied on. Therefore with the equipment known so far the accuracy of the measurement in the course of the measuring period continuously deteriorates, since the spectrum signal characteristic of the intensity of the radiation to be measured is superimposed onto a nearly saw-tooth shaped time-dependent zero level signal.